1. Technical Field
The invention relates generally to safety shut-in systems employed during testing or other operations in subsea wells. More particularly, the invention relates to a safety shut-in system having a valve for trapping fluid under pressure in a pipe string.
2. Background Art
Offshore systems which are employed in relatively deep water for well operations generally include a riser which connects a surface vessel's equipment to a blowout preventer stack on a subsea wellhead. Offshore systems which are employed for well testing operations also typically include a safety shut-in system which automatically prevents fluid communication between the well and the surface vessel in the event of an emergency, such as when conditions in the well deviate from preset limits. Typically, the safety shut-in system includes a subsea test tree which is landed inside the blowout preventer stack on a pipe string. The subsea test tree generally includes a valve portion which has one or more normally closed valves that can automatically shut-in the well. The subsea test tree also includes a latch portion which enables the portion of the pipe string above the subsea test tree to be disconnected from the subsea test tree.
The subsea test tree may be used in conjunction with a retainer valve and a bleed-off valve. The retainer valve is commonly arranged in the pipe string to prevent fluid from being dumped from the pipe string into the riser when the pipe string is disconnected from the valve portion. The bleed-off valve allows controlled venting of pressure that may be trapped between the closed retainer valve and the closed valve portion of the subsea test tree. Generally, the subsea test tree, the retainer valve, and the bleed-off valve are controlled by fluid pressure in control lines which extend from a pressure source on the vessel to the subsea test tree, the retainer valve, and the bleed-off valve.
The retainer valve may be a normally-open or fail-safe-open retainer valve or may be a normally-closed or fail-safe-close retainer valve. When pressure is lost in the control line connected to the retainer valve, a fail-safe-open retainer valve defaults to the open position while a fail-safe-close retainer valve defaults to the closed position. For a fail-safe-close retainer, if the retainer-valve control line is inoperable, e.g., if the retainer-valve control line is inadvertently severed, the fail-safe-close retainer valve remains closed. However, it may be necessary to re-open the retainer valve to permit other operations to be carried out on the well, e.g., kill the well or retrieve a portion of a tubing or wireline which was severed when the retainer valve was closed. Thus, it would be desirable to provide a secondary means through which the retainer valve can be opened if the retainer-valve control line is inoperable.
Conventionally, three control lines are provided to operate the valve portion of the subsea test tree, the latch portion of the subsea test tree, the retainer valve, and the bleed-off valve. However, conventional systems do not allow for independent control of the valve portion of the subsea test tree, the latch portion of the subsea test tree, the retainer valve, and the bleed-off valve. Typically, the valve portion, the latch portion, and the retainer valve have their own dedicated control lines, and fluid pressure in one of the three control lines operate the bleed-off valve. For example, it is common to connect the control line that operates the latch portion to the bleed-off valve such that fluid pressure in the latch control line opens the bleed-off valve to vent pressure trapped between the retainer valve and the valve portion before the latch portion is disconnected from the valve portion. To allow independent control of the retainer valve, the valve portion of the subsea test tree, the latch portion of the subsea test tree, and the bleed-off valve, an additional control line may be provided to operate the bleed-off valve, but this would generally result in incompatibility with existing equipment. Therefore, it is desirable to provide a method for independently controlling the operation of the valve portion of the subsea test tree, the latch portion of the subsea test tree, the retainer valve, and the bleed-off valve using three control lines.